1. Field of the Invention
This invention relates to time-of-flight mass-spectrometers with gasphase ion sources of any number of electrodes.
In a time-of-flight mass-spectrometer a point in time is defined, called start-time, when a group of ions is started on their path. At the end of a drift space the time is measured which an arriving ion has needed on its flight and this time is used to determine the mass of that ion.
The extraction volume is that region within the ion source of the mass-spectrometer, from which, upon start-time, ion paths lead to the surface of the detector of the time-of-flight mass-spectrometer.
The start-time of time-of-flight analysis can be given by:
the point of time, when neutral particles of a gas are ionized within the extraction volume by a laser or electron beam crossing it. PA1 the point of time when the electrode voltages of the ion source are switched on. This is usually the case when ions are to be analysed, since ions can only reach the extraction volume, when the voltages on the electrodes of the ion source are switched off. PA1 1. Scattering events that change the velocity or direction of the ions so strongly such that they do not reach the detector any more. As long as this type of scattering event occurs only for a small part of the ions, the dynamic range and sensitivity will not be significantly impaired. PA1 2. Scattering events that change velocity and direction of the ions only in small amounts, such that they still arrive on the detector, but at incorrect times. These scattering events impair the sensitivity just as little as the first kind of scattering events. The dynamic range is the quotient (correctly arriving)/(incorrectly arriving)ions, the number of incorrectly arriving ions being in the denominator of that quotient. For that reason, this type of scattering event has a very strong influence on the dynamic range of the mass-spectrometer.
As an auxiliary function, it is possible to detect electrons created in a time-of-flight mass-spectrometer. An extraction volume can be defined by analogy. It is not necessary that the extraction volume for ions and the extraction volume for electrons are identical, even though these volumes will at least partly overlap each other. Usually electrons and ions will be drawn out from the source in opposite directions.
The significantly more common case is the detection of ions, and for that reason only that case will be discussed from here on. However, when discussing ions and their paths, the same facts will apply in proper analogy to electrons and their paths.
In any case, there will be within the ion source a first phase of acceleration after start-time. In many cases the ions will be accelerated within the ion source to their final velocity. It is possible, that the ion source also has electrodes for focusing the ions reaching the detector. It can also be the case, that the electrodes for focusing are placed separately, i.e. the ions reaching the detector leave the source with a velocity and coordinate distribution that is not suitable for the further transport through the mass-spectrometer. In that case separate focusing is necessary.
A high particle density in the extraction volume at start-time is of advantage because the number of particles arriving at the detector is proportional to that density. Thus, the size of the extraction volume and the particle density within is a direct measure for the sensitivity of the time-of-flight mass-spectrometer.
Another important attribute of quality for a time-of-flight mass-spectrometer is its dynamic range. The dynamic range is defined here as the factor, by which the signal of some specific mass is allowed to be smaller than other masses without being buried by ions of these other masses that arrive at incorrect times.
Both of these quality attributes will be impaired by scattering of ions on their path to the detector. Two types of scattering events should be distinguished:
The number of scattering events of molecules or atoms with ions on their path to the detector is proportional to the residual gas pressure of the respective regions on the path.
To achieve a high sensitivity of the time-of-flight mass-spectrometer, it is necessary to achieve a high particle density in the extraction volume. To achieve a high dynamic range of the time-of-flight mass-spectrometer, it is necessary to obtain the lowest possible residual gas pressure. A high particle density in the extraction volume will increase the amount of unwanted gas ballast, said gas ballast increasing the residual gas pressure. In many applications of time-of-flight mass-spectrometry on gasphase particles this will be a problem, if it is desired to optimize both attributes of quality simultaneously.
Usually a time-of-flight mass-spectrometer will be separated into regions of different pressures, ordered with sinking pressure from the sample introduction, i.e. the generation of the analyte gas or ion beam, to the ion source, along drift space in the time-of-flight mass-spectrometer. In order not to obstruct the analyte gas or ion beam, nor to obstruct the ions on their paths to the detector, adjacent regions have to be connected by flow restrictions. Such a construction will allow a high particle density in the extraction volume, at the same time guaranteeing a low residual gas pressure, i.e. a low scattering probability in the drift space of the time-of-flight mass-spectrometer.
Flow restrictions are understood here as openings of small cross section, that are large enough to pass ions unhindered on their way to the detector. However, their conductivity for gases should be significantly lower than the pumping capacity of the pump for the region of lower pressure.
The most basic implementation of a flow restriction is an opening or aperture of some cross section in a plane separating regions of different gas pressure. However, tubes or constructions with tube character have a significantly lower conductivity for gases than openings in a plane and will be often preferred.
Skimmers are cones with an opening in the tip facing the gas beam. Skimmers have a similar conductivity for gases as openings in a plane and should preferentially be used, if the gas beam has a high pressure.
2. Description of the Related Art
From the publication of Michael et al. (Review of Scientific Instruments, volume 63(10), pages 4277-4284, 1992) it can be inferred that the time-of-flight mass-spectrometer is divided into regions of different pressures. The region that includes the extraction volume has a higher pressure than parts of drift space. However, as can be inferred from part C "TOF operation", the ion source, the flow restriction and the focusing electrodes are individual units, arranged separately. ("A restriction of 1 in. tubing is placed between the flight tube and the main chamber").
Arranging the ion source and the flow restriction separately has the disadvantage, that ions have to move a comparatively long way through the dense gas of the ion source and thus the probability of scattering with residual gas particles is large. Aside from that, the difference in pressure between the two regions is just somewhat less than a factor of 4. Thus it seem that either the diameter for this flow restriction has been chosen too large or its length has been chosen too small.
The German patent application DE 41 08 462 A1 and the publication of Rohwer et al. (Zeitschrift fu/ r Naturforschung, volume 43a, pages 1151-1153, 1988) show a skimmer that is arranged separate from the ion source. Here the distance between skimmer opening and the extraction volume is comparatively large.
This comparatively large distance is of disadvantage for the following reasons: It is desired that the analyte gas or ion beam crosses the extraction volume, because from here ions start on their path into the mass-spectrometer. If parts of the analyte gas or ion beam do not cross the extraction volume, said parts do not enhance the sensitivity, they only increase the residual gas pressure. The increased residual gas pressure reduces the dynamic range of the time-of-flight mass-spectrometer. The analyte gas or ion beam is always more or less divergent, so with increasing distance skimmer/extraction volume the portion of said gas or ion beam that does not cross the extraction volume becomes larger. This large distance has the disadvantage that with highly loading the ion source with gas, causing a high residual gas pressure, only a low particle density in the extraction volume results. This will cause a reduced sensitivity and a reduced dynamic range of the time-of-flight mass-spectrometer.